Method of drilling and completing a well in an unconsolidated formation

ABSTRACT

The invention provides a method for drilling, completing, producing and conducting other well operations in a manner to avoid or limit formation failure of an unconsolidated formation of interest. Thus, a determination is made of the minimum pressure which must be exerted in the borehole against such formation to result in a radial stress on the borehole wall of not less than that minimum value required to maintain the Mohr circle to be within the Mohr failure envelope for such formation. Then at least this minimum pressure is deliberately maintained on the formation during all operations conducted in it. Thus, even after completion, a pressure is permanently exerted on the formation such that arch formation can result thereby reducing or eliminating sand production while the well is being produced or treated or while it is being used as an injection well or for other purposes.

[ Sept. 23, 1975 Q1 3 9 3 0 7 9 ll 34- Umted States Patent [191 Suman, Jr.

[ METHOD OF DRILLING AND COMPLETING A WELL IN AN UNCONSOLIDATED FORMATION [76] Inventor: George 0. Suman, Jr., 3701 Kirby Dr., Suite 55 8, Houston, Tex. 77006 [22] Filed: Sept. 23, 1974 [21] Appl. No.: 508,031

Related U.S. Application Data [63] Continuation-in-part of Ser. No. 437,231, Jan. 28,

[52] U.S. Cl. 166/250; 175/50; 175/65 [51] Int. Cl. E21B 43/00; E2113 49/00 [58] Field of Search 166/250, 305 R, 278;

[56] References Cited UNITED STATES PATENTS 3,399,723 9/1968 Stuart 175/57 X 3,434,540 3/1969 Stein 166/250 3,557,886 6/1969 Cobbs 175/50 3,587,741 6/1971 Casey 166/278 3,670,829 6/1972 Overton 175/50 3,695,355 10/1972 Wood et a1. 166/278 X 3,750,766 8/1973 Thompson et a1. 175/50 OTHER PUBLICATIONS Hall, Jr. et al., Stability of Sand Arches: A Key to Sand Control, Journal of Pet. Tech, July, 1970, pp. 202-210.

Rawlings, Jr., Use of Hydraulic Fracturing Equipment for Formation Sand Control, Journal of Petroleum Tech., May, 1958, pp. 2932.

Primary Examiner-Stephen J. Novosad Attorney, Agent, or FirmW. F. Hyer; Marvin B. Eickenroht [57] ABSTRACT The invention provides a method for drilling, completing, producing and conducting other well operations in a manner to avoid or limit formation failure of an unconsolidated formation of interest. Thus, a determination is made of the minimum pressure which must be exerted in the borehole against such formation to result in a radial stress on the borehole wall of not less than that minimum value required to maintain the Mohr circle to be within the Mohr failure envelope for such formation. Then at least this minimum pressure is deliberately maintained on the formation during all operations conducted in it. Thus, even after completion, a pressure is permanently exerted on the formation such that arch formation can result thereby reducing or eliminating sand production while the well is being produced or treated or while it is being used as an injection well or for other purposes.

9 Claims, 2 Drawing Figures 2500 0422 00 0 0000002005? it 2000 a 02/000005 J/l/VD RfG/OA/ I :3 500 0/20/70 P 29L ALGMNE a /000 42.2 TRAIVJ/T/O/V I 020/0002 {5 I 72 0001/0/00 m I /4 /5 0 t /7 NUAM/M JT/PEJJ, ,oJz'

METHOD OF DRILLING AND COMPLETING A WELL IN AN UNCONSOLIDATED FORMATION This application is a continuation-in-part of my copending application, Ser. No. 437,231, filed Jan. 28, 1974.

This invention relates to a method for drilling, com pleting, producing and conducting other well operations in such a manner as to minimize or eliminate the production of sand and in a manner that a formation of interest is maintained in a machanically stable condition.

In the production of oil and gas from wells, the problem of preventing the flow of sand from unconsolidated formations into the well has been long existing with its significance varying from formation to formation and with depth. Various solutions have been proposed to alleviate this problem such as consolidation of the for mation by chemical treatment or by the use of gravel packs and sand screens to physcially impede the flow of sand into the well. However these proposed solutions have left much to be desired primarily because of their unreliability. Failure to satisfactorily solve the sand production problem may be due, in part, to a lack of understanding of the fundamental factors involved in the sand production mechanism and due also to the many perplexing and seemingly contrary aspects of the problem. While it is patently obvious that sand production is a result of formation failure, the underlying reasons for such failure and the application of a mechanism to prevent such failure has thus far eluded engineers and scientists although work has been done in an attempt to define the failure mechanism. Some recent work in this area is that of Hall and Harrisberger as published in the Journal of Petroleum Technology, July 1970, pp. 831-829. This paper presents an extension of prior soil mechanics failure studies at low stress levels to failure studies of well sands at considerably elevated stress levels and develops the familiar Mohr envelopes for several typical unconsolidated oil well sands. The instant invention involves a further extension of the Hall, et al work and provides a new concept for applying forces in a borehole in such a manner as to permit sand control and to achieve other advantages.

It is an object of this invention to provide a method for sand control by establishing states of stress in an unconsolidated formation which will prevent or at least reduce sand production.

It is another object to provide a method for drilling and completing an oil or gas well in an unconsolidated formation wherein the radial force exerted on the walls of a borehole penetrating such formation during drilling and completion is maintained so as to avoid or limit formation failure.

Another object is to provide such a method which not only so avoids or limits formation failure but which provides a proper stress environment in the formation for sand arch stability so that the formation during production or treatment tends to be mechanically stable so as to avoid or reduce sand production.

In the drawings FIG. I is a reproduction of a Mohr diagram corresponding to FIG. 4 of the above Hall, et al article; and

FIG. 2 illustrates a well completed in accordance with one aspect of this invention.

Referring to FIG. I, the Mohr diagram shown therein is, as is well known to those skilled in this art, of the type which indicates that failure of sand will occur when the major and minor principal stresses 0', and 0' combine to produce a criticaland characteristic value in the ratio of shear stress to normal stress in the plane of shearing. Thus the shear strength of the sand is described by the Mohr envelope and its slope is the angle (alpha) of shearing resistance.

Thus, in FIG. 1, the Mohr envelope consists of a linear portion 10, another linear portion 11 of less slope than that of portion 10 and a transition region 12 indicated by the dashed line connecting the two linear regions. The Mohr circles l3, l4 and 15 are part of a family of circles which define the Mohr envelope. In any given formation, when the major principal effective stress at 16 has a relation to the minor principal effective stress at 17 to be such that the resulting Mohr circle is tangent to or lies beneath the Mohr envelope, it can generally be stated that the formation is stable. However, if the relationship of these major and minor effective stresses are such that the resulting Mohr circle exceeds the envelope, the formation will be in a state of failure.

The first linear portion 10 of the envelope which defines Region I is one in which, when the Mohr circle exceeds that portion of the envelope, the failure of the formation will be by dilation in that there will be a rolling sliding motion between the grains of sand characterized by an increase in porosity in the failed portion of the formation. In Region III, failure occurs by shearing or crushing of the sand grains. ln transition Region II, there'is a combination of failure by dilation and shearing.

In an undistrubed formation, there exists a system of triaxial stresses, a vertical effective stress o and two effective horizontal stresses (o'hd H), and in the case of the latter, one may be less than the other. In the normal state, these stresses will be in a state of balance in the sense that there is no formation failure. However, when a borehole penetrates the formation, this balance may be disturbed to the extent that formation failure occurs, the degree of which for any given formation will be dependent in large part on the stress pattern at or near the borehole wall. Thus when the borehole penetrates the formation, the stress pattern controlling failure will be determined by the major principal effective formation stress and the minor principal effective stress at the borehole wall, the latter commonly being the radial stress (o on the wall due to the presence of drilling fluids or cementation fluids. Another stress which will exist at the borehole wall will be the tangential or hoop stress (0-9). Normally, an increase in the radial stress will result in a decrease in the tangential stress and vice versa. Reference is made to the article entitled Mechanics of Hydraulic Fracturing by Hubbert and Willis in Petroleum Transactions, AIME, Volume 210, pp. 153-166, 1957, for a further discussion of the relationship of the horizontal effective stresses, including the hoop stress and their further relation to hydrostatic and pore pressures.

In accordance with this invention, a determination is made of the minimum pressure which must be exerted in a borehole against an unconsolidated formation which will result in a radial stress on the borehole wall and in the immediately surrounding formation which will be of a value not less than the minimum value required to maintain the Mohr circle as defined by such radial stress and the major principal effective formation stress of the formation to be within the bounds of the Mohr failure envelope for such formation. After this determination has been made, the pressure exerted on the borehole wall is deliberately maintained to be sufficient to assure that at least the minimum radial stress as aforesaid is exerted at all times on the borehole wall during drilling and completion operations in the formation.

Stated in another manner, the difference between the major principal effective stress of the formation and a minor principal effective stress of such formation as defined by the Mohr circle at incipient failure is determined. Then the minor principal effective stress at the borehole wall is deliberately maintained at a value such that the difference between it and the major principal effective stress of the formation does not exceed the determined difference. For example, if the Mohr circle of incipient failure is as shown in FIG. 1 at 15, the minor stress at the borehole wall will be maintained at at least 750 psi to prevent formation failure by shearing or crushing and preferably at a higher value, the magnitude of which may be determined by other well conditions such as potential lost circulation problems. Similarly if the Mohr circle of incipient failure is as shown at 14, the minor stress will be maintained at at least 500 psi to avoid failure in the transition region by a combination of dilatant and shear-crushing failure. If the Mohr circle is as shown at 13, the stress will be maintained at at least 250 psi to avoid dilatant failure. It can be shown that the tangential, vertical and radial effective stresses at the well bore wall are related at failure by a factor which is the extreme ratio which can exist between the major and minor principal effective stresses as follows:

I +sin alpha K l sin alpha wherein alpha is the angle of internal friction (shearing resistance). Thus, for the miocene sand of FIG. 1, and for the particular angles of internal friction shown thereon, the K values for the crushing range and the dilatant range are 3 and 5, respectively. Angles for other sands can be readily determined using routine procedures known to those skilled in the art.

Thus it will be seen that when the major principal effective stress of a formation in such that an excessively low radial stress causes formation failure by dilation, such low radial stress will be less than one-fifth of the major principal stress. When operating in this region the radial stress must be maintained to be greater than one-fifth of the major principal effective stress in order to avoid formation failure by dilation. Similarly, when the formation being drilled has a major principal effective stress such that an excessively low radial effective stress would cause failure in the crushing region, such failure can be prevented by maintaining the radial effective stress to be greater than one-third of the major principal effective stress. Of course, when drilling in a formation having a major principal effective stress causing the Mohr circle to fall in the transition region, the minimum radial effective stress to prevent failure will be a value between one-fifth and one-third of the major principal stress depending upon the exact point in the transition region one is operating.

It has been conceived that failure to maintain the radial stress above the particular minimum as above discussed, there will exist an annular section of the formation immediately surrounding the borehole which has failed by dilation. Surrounding this will be second annular portion which has failed by a combination of dilation and shear failure. Still a third annular portion will surround the second wherein the formation will have failed by shear failure. Thus referring to FIG. 1, if the radial stress is less than the minor principal stress 17, the formation will fail by pure dilation. As this inner portion of the formation fails, this reduces the radial stress on the next annular portion so that it fails as indicated above and so on. The above assumes, of course, that the major principal effective stress is sufficiently large to cause shear failure.

The maintenance of the prescribed radial stress can be accomplished by using suitably weighted drilling fluids and cementing fluid which can be augmented by such techniques as drilling under pressure, etc. In this connection, it becomes necessary to consider the interstitial fluid pore pressure of the formation. Thus, the pore pressure must be subtracted from the fluid pressure to determine the net radial effective stress beingapplied to the formation. In fact, the effective stress in any of the three triaxial directions will be equal to the total stress minus the pore pressure and later changes in pore pressure will cause corresponding stress changes.

The determination of the vertical effective principal stress in an undisturbed formation can be by procedures well known to those skilled in the art. Thus density logs can be used to determine overburden pressure in formations that have not dilated during drilling and this less the pore pressure yields the vertical principal effective stress. The undisturbed formation will also be stressed by a minor horizontal effective stress and another horizontal stress usually having a value either equal to the minor horizontal stress or between the minor horizontal stress and the major vertical stress. The radial stress is equal to the mud or cement weight less pore pressure. Ordinarily, it will be desirable to maintain the radial effective stress at a value below the minor horizontal effective principal stress in order to avoid or minimize the likelihood of fracturing the formation. The minor horizontal effective principal stress can normally be determined for unconsolidated formations by the following relationship:

wherein:

Fp is the fracture injection pressure; and

p is the formation pore pressure.

The above relationship for determining 0" can be utilized when the minor effective principal stress is horizontal rather than vertical which is the case in tectonically relaxed and young geological sediments in which unconsolidated sands are commonly found and Fp is determined for the formation when the latter has not been fractured during drilling operations and has not failed in the plane of fracture because of inadequate radial stress application as explained above, or for other reasons.

Thus the radial effective stress should be maintained to be above the minimum as dictated by the appropriate Mohr diagram as explained above but less than that to cause fracturing of the formation. While the desired wherein Fp,and p and K are as defined above and S is the overburden pressure.

The original, undisturbed vertical stress may normally be assumed equal to the overburden, the least horizontal stress equal to fracture pressure (when fracture pressure is less than overburden pressure) and the greatest horizontal stress either equal to the least hori zontal stress or some logical deviation from the horizontal and vertical stresses based upon geologic reasoning. In those formations where the greatest horizontal stress deviates substantially from the least horizontal stress as shown by field experience and/or geologic considerations, the radial stress would be adjusted accordingly, preferably by maintaining the radial effective stress at a value closer to that at which fracturing of the formation occurs.

As indicated above, in order to avoid formation failure of any kind, the radial effective stress on the borehole wall should be not less than that value required to maintain the Mohr circle as defined by such radial stress and the maximum principal effective stress of the formation to be within the Mohr failure envelope for such formation throughout Regions I, II and III. However, in some cases, it may be sufficient and perhaps even desirable, for either economic or other reasons, to apply a radial stress which is sufficient to avoid dilatant failure in Regions l and II but insufficient to avoid all shear crushing failure in Region III. Thus, referring to FIG. 1 for example, the maintenance of at least 750 psi radial stress will avoid dilatant failure regardless of the magnitude of the maximum principal effective stress. If the latter is less than about 2250 psi, FIG. 1 indicates there will be no Region III crushing failure. However, if the maximum stress is greater than 2250 psi, the maintenance of 750 psi will only result in the formation failing by Region III shear crushing action until the stress state of the formation at some point outwardly of the borehole wall stabilizes at 2250 psi and thereafter there will not be any further failure.

It is also in accordance with one aspect of this inven tion that a method is provided for not only avoiding or minimizing formation failure as the borehole is being drilled through and a completion made in it, but also to permit stabilized sand arches to form to prevent or minimize sand production. As used herein, the term arch or sand arch" denotes a configuration of stressed sand particles spanning an opening and serving to support a load by resolving the stress acting in a direction parallel to the axis of the opening into stresses acting in a direction transversely of the opening. This configuration may be in the shape of a curved structure spanning the opening and thus consisting of an arch of sand grains locked together by stresses applied thereto sufficient to cause such locking or stabilization but insufficient to cause the arch to fail. Any grains of sand originally between the arch and the opening may remain in that position or may flow through the opening. Since sand from an unconsolidated formation must flow into the well through an opening, it can be seen that the establishment and preservation of stable sand arches over the hole would reduce or minimize the production of sand. Similarly, the establishment of stable sand arches around holes out in the formation, such as those between proppants injected during fracturing of a formation or those between grains of gravel used in gravel packing should increase the efficiency of the fracturing process or gravel packing process.

In accordance with this invention, the effective radial stress on the formation is made to be sufficiently high that arch formation can result with consequent reduction or elimination of sand production while the well is being produced or treated or while it is being used as an injection well or for other purposes.

In general, the effect of arch load can be divided into four ranges as follows (the term arch load is used herein to designate those stresses which act on the arch in a direction to tend to collapse it):

Range 1: In this range, the arch load is quite low so that either an arch does not form or a very tenuous arch forms. The sand is in a stress state permitting dilatant action of the sand body and the sand will flow due to drag forces caused by flow of fluid in a rolling sliding motion through an opening large enough to prevent bridging.

Range 2: As the arch load increases, a rate sensitive arch will form. In this type of arch, the stability is a function of flow of fluid therethrough and its degree of restraint is sensitive to interfacial tension and sand grain angularity. In this region, with lower or no flow rates, the state of stress (arch load) is still high enough to prevent dilatant action on the sand of the arch, but cannot do so when the flow rate becomes sufficiently high. I-Ience arch failure can occur at such high flow rates with expansion of the inner row of sand grains and rolling sliding motion between the grains.

Range 3: In this range, the arch load is increased to a point where the sand grains are held in an interlocked condition unable to dilate for the rolling sliding type of failure. This type of arch is, for all practical purposes, insensitive to the rate of flow of fluid through it and the failure of this arch must be accomplished by sand grain shearing. The range can be described as one of a rate stabilizing arch in the sense that the increases in load associated with increased drawdown or decreased pore pressure may further stabilize and strengthen the arch. In this range, grain angularity contributes to the degree of restraint or stability of the arch.

Range 4: In this range, the arch loads are sufficiently high that the arch will fail by crushing the inner row of sand grains. Sand grain crushing accompanied by the failure of sand arches with the production of minor amounts of sand would shift the arching load back toward the more stable condition of range 3. This is due to the relative massive extent of the reservoir which is available to adjust and maintain the load on the arches within range 3.

Referring to FIG. 1, the minimum radial stress for maintaining a formation in a stress state such that a range 3 type arch will form will be the minor (radial) principal effective stress which is defined by the Mohr failure circle for the initial portion of region I. In determining the desired radial stress, the pore pressure must be taken into account in that it must be substracted from the applied stress to arrive at the desired radial stress. For example, with the particular sand of the Mohr diagram and for curve of FIG. 1, the radial effective stress could be selected to be about 750 psi or at a lower value when the maximum effective stress is less as at shallower depths. Then as pore pressure decreases due to flow from the formation or for other reasons, the arch load would increase to further stabilize the formation. Conversely, for injection wells, the initial radial effective stress can be made considerably higher so that as fluid is injected and pore pressure increases, the resultant decrease in effective radial stress will not be sufficient to permit arch failure.

There are a number of manners in which the desired radial stress can be exerted against the formation of interest after the well has been completed. In some instances, it may be possible to adjust the height of the cement and its density to achieve the desired radial stress. However, this mode may be inapplicable to many situations for a number of reasons including the problem of entrapping mud between the column of cement and the formation which can drain away during production and relieve stress on the formation. A more preferred mode is shown in FIG. 2 which is shown and described in detail in the above identified co-pending application. Therefore, the disclosure of such application is incorporated herein by reference. In making this type of completion, and while the borehole is filled with drilling mud exerting the required radial stress on the formation, a string of casing is run into the hole after a packer, preferably inflatable packer 21, has been made up therein and spaced so that when the casing has been landed, the packer will be opposite the formation of interest. The well can then be subjected to a primary cement job by conventional techniques so that cement 22 fills the annulus between the casing string and the wall of the borehole. During this operation, packer 21 remains in deflated condition so that cement can flow therepast to cement the intervals above and below the packer.

The packer is preferably of the inflatable type and can be of the construction shown on page 2847 of the 1972-73 Composite Catalog 0 f Oil Field Equipment and Services with suitable ajustment in its length to accommodate formations of various thicknesses. Other inflatable packer constructions can be used. In any event, the packer usually comprises a mandrel 23 which can be provided by a length of casing sealingly and slidably extending through an upper connector assembly 24 and connected to a lower connector assembly 25. A radially expansible element 26 extends between and is connected to the upper and lower connector assemblies 24 and 25. Expansion of the packer is accomplished by forcing cement through passageway 27 into the interior of the sleeve until the latter is expanded to a degree to exert the desired radial stress on the formation, allowing for any contraction or expansion of the cement after it has been set. A valve system 28 can be provided to open at a preset pressure to allow cement to flow into the packer and to close to prevent backflow of cement from the packer before the cement has set. Such a valve system is indicated in the above page of the Composite Catalog.

During the primary cementing operation, a suitable plug 29 or other shutoff device is placed in the casing and pumped down the same by additional cement on top of the cementing plug. As the cementing plug passes the packer, it will shear off a plug 30 so that cement is caused to flow into the packer to inflate and expand the same. This causes the packing element or sleeve 26 to not only displace the primary cement between the packing element and the formation but also to displace any mud or other well fluid that may exist in such an interval so that there is a continuous and uninterrupted contact of the packing element with the formation face. After the packer has been set, cement remaining in the casing can be drilled out or otherwise removed in a conventional manner.

After the cement in the packer has set to a solid mass, one or more performations 31 are formed from the interior of the element 26 into the formation as shown.

While the perforations have been referred to as beingformed, such as by conventional shaped charge perforating techniques, it is possible to form the perforations by using a Permeator of the type sold by the Permeator Corporation of Houston, Texas. In using Permeators, their housings are screwed into threaded openings in the packer mandrel and their smaller pistons are attached to openings in the packing element. Then after the packer has been expanded as aforesaid, the acid soluble plugs in the Permeators can be dissolved in conventional fashion to open the perforation from the interior of the packer to the formation.

By using the technique just described, it will be seen that the radial pressure to be exerted on the formation can be controlled within the limitations of the equipment being used.

From the foregoing it will be seen that this invention is one well adapted to attain all of the ends and objects hereinabove set forth, together with other advantages which are obvious and which are inherent to the method.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

As many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed l. A method drilling and completing a well having a borehole penetrating an unconsolidated formation comprising the steps of determining the minimum pressure which must be exerted in the borehole against such formations to result in a radial effective stress on the borehole wall of not less than that value required to maintain the Mohr circle as defined by such radial stress and the maximum principal effective stress of the formation to be within the Mohr failure envelope for such formation throughout at least Region I of said envelop; drilling said borehole into such formation; running a tubular member into the well; establishing fluid communication between said tubular member and said formation; and deliberately maintaining at least said minimum pressure continuously against said formation during all of said drilling, running and establishing steps.

2. The method of claim 1 wherein said radial stress is of a value to maintain the Mohr circle within at least Regions 1 and II of the Mohr failure envelope.

3. The method of claim 1 wherein said radial stress is of a value to maintain the Mohr circle within Regions 1, II and III of the Mohr failure enevlope.

4. The method of claim 1 wherein the radial stress is of a value to maintain the Mohr circle within Region ll] of the Mohr failure envelope.

5. The method of claim 1 in combination with the further steps of producing fluid from said formation through said fluid communication into the tubular member while maintaining at least said pressure against the formation.

6. The method of claim 1 wherein the establishing steps includes establishing a seal between the tubular member and said formation having a fluid passageway therethrough, said seal maintaining at least said minimum pressure on said formation.

7. The method of claim 6 wherein said seal is established by radially expanding a packer into contact with said formation by forcing cement into said packer until the latter exerts at least said minimum pressure on said formation.

8. The method of claim 6 wherein said fluid passageway is formed by perforating the packet and the cement therein after the latter has at least partially solidified.

9. A method of drilling and completing a well having a borehole penetrating an unconsolidated formation comprising the steps of determining the minimum pressure which must be exerted in the borehole against such formation to result in a radial effective stress on the borehole wall of not less than the fraction defined y l sin alpha l sin alpha said drilling running and establishing steps. 

1. A method drilling and completing a well having a borehole penetrating an unconsolidated formation comprising the steps of determining the minimum pressure which must be exerted in the borehole against such formations to result in a radial effective stress on the borehole wall of not less than that value required to maintain the Mohr circle as defined by such radial stress and the maximum principal effective stress of the formation to be within the Mohr failure envelope for such formation throughout at least Region I of said envelop; drilling said borehole into such formation; running a tubular member into the well; establishing fluid communication between said tubular member and said formation; and deliberately maintaining at least said minimum pressure continuously against said formation during all of said drilling, running and establishing steps.
 2. The method of claim 1 wherein said radial stress is of a value to maintain the Mohr circle within at least Regions I and II of the Mohr failure envelope.
 3. The method of claim 1 wherein said radial stress is of a value to maintain the Mohr circle within Regions I, II and III of the Mohr failure enevlope.
 4. The method of claim 1 wherein the radial stress is of a value to maintain the Mohr circle within Region III of the Mohr failure envelope.
 5. The method of claim 1 in combination with the further steps of producing fluid from said formation through said fluid communication into the tubular member while maintaining at least said pressure against the formation.
 6. The method of claim 1 wherein the establishing steps includes establishing a seal between the tubular member and said formation having a fluid passageway therethrough, said seal maintaining at least said minimum pressure on said formation.
 7. The method of claim 6 wherein said seal is established by radially expanding a packer into contact with said formation by forcing cement into said packer until the latter exerts at least said minimum pressure on said formation.
 8. The method of claim 6 wherein said fluid passageway is formed by perforating the packer and the cement therein after the latter has at least partially solidified.
 9. A method of drilling and completing a well having a borehole penetrating an unconsolidated formation comprising the steps of determining the minimum pressure which must be exerted in the borehole against such formation to result in a radial effective stress on the borehole wall of not less than the fraction defined by 